In recent years, cold cathode fluorescent lamps (CCFLs) and light emitting diodes (LEDs) have been widely used. In comparison with the common incandescent lamps, LEDs or CCFLs have an increased illuminating efficiency and an extended service life. With the maturity of the LED and CCFL technology, LEDs or CCFLs will replace all conventional lighting facilities. Until now, LEDs or CCFLs are widely used in many aspects of daily lives, such as household lighting device, automobile lighting devices, handheld lighting devices, backlight sources for LCD panels, traffic lights, indicator board displays, and the like.
Generally, the cold cathode fluorescent lamp or the light emitting diode is driven to illuminate by a light source driving circuit. In addition, the brightness value of the cold cathode fluorescent lamp or the light emitting diode is controlled by the light source driving circuit. Based on the persistence of vision, the cold cathode fluorescent lamp or the light emitting diode is alternately turned on and turned off so as to intermittently emit light under the circumstance imperceptible to the human beings.
The conventional light source driving circuit includes a control circuit, a transformer and a switching circuit. The control circuit generates a control signal. According to the control signal, the switching circuit is alternately conducted or shut off. As such, the utility power received by the primary winding assembly of the transformer is converted into a regulated voltage, which is transmitted from the secondary winding assembly of the transformer to the cold cathode fluorescent lamp or the light emitting diode. Moreover, according to a brightness adjusting signal, the control circuit will control the duty cycle or the switching frequency of the switching circuit. Generally, the brightness adjusting signal includes alternate enabling signal and disabling signal. In response to the enabling signal, the cold cathode fluorescent lamp or the light emitting diode illuminates. In response to the disabling signal, the cold cathode fluorescent lamp or the light emitting diode is turned off. As the duty cycle or the switching frequency of the switching circuit is changed, the regulated voltage transmitted from the secondary winding assembly of the transformer is altered. As the regulated voltage transmitted from the secondary winding assembly of the transformer is altered, the time period of turning on or turning off the cold cathode fluorescent lamp or the light emitting diode will be increased or decreased. According to the brightness adjusting signal, the brightness value of the cold cathode fluorescent lamp or the light emitting diode is adjustable.
Since the control circuit of the conventional light source driving circuit is connected to the utility power through the primary winding assembly of the transformer and the brightness adjusting signal is directly transmitted to the control circuit, the user has a risk of getting an electric shock during the process of operating the brightness adjusting signal. In other words, the electrical safety of the conventional light source driving circuit is unsatisfactory.
Moreover, since the time period of switching the brightness adjusting signal from the enabling signal to the disabling signal or the time period of switching the brightness adjusting signal from the disabling signal to the enabling signal is very short, a problem of causing light source scintillation will occur when the cold cathode fluorescent lamp or the light emitting diode is driven by the conventional light source driving circuit.
There is a need of providing a light source driving circuit to obviate the drawbacks encountered from the prior art.